1. Field of the Invention
The present invention relates generally to the field of audio/video systems and more particularly to apparatus for correcting the output of playback systems to reduce or eliminate the effects of wow and flutter.
2. Description of the Prior Art
Commonly known audio and video disc and tape playback devices exhibit frequency modulation effects, which to a greater or lesser degree, depending upon the quality of the device, distort the output. One type of distortion is in the form of instantaneous frequency deviations introduced by speed fluctuations in the recording and playback processes and is referred to as wow for lower frequency deviation rates and flutter for higher deviation rates.
Another form of frequency modulation distortion is observed in the transmission of short wave radio signals. Such signals are reflected from an atmospheric layer which is constantly changing in altitude about an average altitude. This phenomenon produces a variation in the radio frequency pathlength and thus a doppler shift in the frequency at the receiver. While technically not wow and flutter the character of the distortion is essentially the same.
Prior to the present invention measures taken to minimize wow and flutter have included the use of high inertia drive mechanisms in tape and disc players. Quality playback devices also employ mechanical servomechanisms, however, when provided with sufficient gain to be effective, they tend to become unstable and oscillate thus defeating their purpose. Alternatively, synchronous drive motors are used which, on average lock to the power line frequency but may have short term fluctuations above and below that frequency. Typically these measures are not adequately effective for instantaneous frequency deviations in excess of about 2 Hz. Moreover, mechanical eccentricities in rotating mechanisms can cause frequency modulations that are not controllable through application of a servo to the driving motor.